Rolling method



United States Patent Office 3,118,330 ROLLING METHOD Hans Geipel, Kret'eld, Germany, assignor to Deutsche Edelstahlwerke Aktiengesellschait, Krefeld, Germany No Drawing. Filed Oct. 13, 1959, Ser. No. 846,057 Claims priority, application Germany .Oct. 18, 1958 4 Claims. (CI. 80-60) As known it is a matter of especial difficulty to roll alloyed steels in a satisfactory manner, particularly ferritic chromium steels with and without additions or" nickel. These steels have a pronounced tendency to stick to the rolls, and occasionally this trouble is also known to arise in the case of austenitic chrome-nickel steels or chromium-manganese steels. The result of this tendency is that, on the one hand, the surface of the rolled sheet is not satisfactory and, on the other, that the rolls require frequent regrinding to remove roughness and pitting which may eventually lead to the rolled sheet winding itself round the rolls.

If steps are taken for chilling the rolls between consecutive passes, that is to say if rolling is performed with practically cold rolls, and special precautionary measures are taken, such as the frequent rubbing down of the rolls with emery, then it is possible to roll down even alloyed ferritic steel sheet. However, it is very desirable to perform the rolling process 'with rolls intentionally maintained at a temperature of about 400. The advantage of rolling with warm rolls is that higher rolling pressures can be applied. Rolling can also proceed in quicker succession because the rolls need not be specially cooled each time a plate is rolled down. However, alloyed ferritic steels cannot be rolled by this method at all, because their tendency to stick to the rolls is thereby increased very considerably. Proposals have already been made to treat the rolls with a chemical which is intended to prevent the rolled material from sticking. This proposal requires that the rolls be sprayed with the chemical whenever a small number of passes has been completed. Apart from the expense caused by the consumption of the agent by spraying, the rate of production is reduced in exactly the same way as in the method of chill rolling the sheet, in which the rolls are cooled each time a slab is rolled down.

The object contemplated by the inventor is a method which will permit advantage to be taken of warm rolling for rolling down alloyed ferritic steels, and which will prevent sticking at maximum rate of production without additional substantial cost. According to the invention the problem is solved by producing a special oxide film on the surface of the material to be rolled. As known, for the purpose of rolling alloyed steels, especially ferritic steels, into sheet, an intermediate product, called pickled plate, from which the scale has been removed by pickling, is first produced. The clean plate is then heated to rolling temperature and rolled down into sheets. It is now proposed by the invention to provide this bright pickled plate-aud analogously any other corresponding intermediate product-with an all-over, firmly adherent, oxide film which will then serve as a parting layer between roll and rolled material in the process of rolling down the plate. This oxide film is especially effective if on ferritic steels, with or WithOllt an addition of nickel, it has a brown velvety appearance and despite its adhesion it can be spread by rubbing. This film consists substantially of the oxide of trivalent iron. In the case of a ferritic chrome-steel with about 17% Cr, for instance, an oxide film containing about 70% Fe O about 6% FeO, small quantities of chromium oxide, and a certain content of water, has proved to be very suitable. The effective part of the film for the purposes of the in- Patented Jan. 21, 1954 vention is the Fe O layer which is in direct contact with the bright surface of the steel. It is, of course, not necessary to observe the above composition exactly, but the major proportion of the film must consist of F0 0 The important step therefore consists in submitting the bright intermediate product, for instance a pickled plate, to a thorough surface oxidation. This can be most conveniently done by immersing the pickled clean plate in a known type of oxidising molten salt bath. A thin brown velvety film will then form directly on the bright surface of the plate. The plates, thus provided with an adherent brown oxide film, are then heated in a conventional furnace to rolling temperature and they can then be rolled down into sheets in rapid succession, without risk of sticking to the rolls, at roll temperatures of about 400. The velvety oxide film to some extent protects the steel surface from scaling. However, any scale that does form is innocuous because the previously formed velvety oxide film remains as the top layer and therefore continues to provide an effective parting film between roll and rolled material. The oxide film is essentially a parting layer in hot-rolling to prevent adhesion of the material being rolled to the rolls and not a means for holding a lubricant as has been proposed in connection with drawing ferrous metals through dies.

It is especially convenient to use a salt bath consisting of caustic soda and about 10% of sodium nitrate. Up to 0.5% of borax may be added thereto. The important feature is the presence of a sufficient amount of an oxygen-supplying medium. Although other oxidants, such as perchromate, are effective, sodium or potassium nitrate have been found to be the most useful. However, the presence of other substances, such as of chromates, carbonates, or the like, is by no means harmful. The both should be in the molten state at temperatures around 400 C. If the bath has the preferred composition this desideratum will be assured by the presence of the caustic soda. After thorough immersion for which a period of 10 min. should be sufiicient, and after allowing the melt to drip off, the plate is sprayed with water and rinsed. The resultant thin adherent brown oxide film remains, and it also persists when the plate is put into the furnace for heating.

According to one embodiment of the invention:

A ferritic chromium steel plate or slab is obtained with a chromium content of about 17%. This is intended to be rolled to the sheet. It is pickled by any suitable conventional method to remove the scale. Normally it is then rolled. According to the invention the clean plate is immersed in a bath consisting of caustic soda, about 10% of sodium nitrate and up to 0.5% of borax. The bath is in the molten state at a temperature of about 400 C. The clean plate is immersed for -a period of about 10 minutes and a velvety brown oxide film containing about 70% Fe O is formed which can be spread by rubbing or rolling. The melt is allowed to drop off and the plate is sprayed with water and rinsed. 'The film remains. The plate is then heated in a furnace to rolling temperature and rolled at about 400 C. when the film forms a parting layer between the rolls and the steel. The film can be subsequently readily removed.

The preferred salt bath is especially advantageous because baths of this and similar composition are usually available in any event in a rolling mill plant which handles alloyed steels where they are used to transform the firmly adherent black scale on the steel into a state in which it is more readily removable by pickling.

The steps proposed by the invention also offer certain advantages in connection with chilled roll procedures in which the rolls are maintained at lower temperatures. They are also applicable to austenitic steels which occasionally likewise tend to stick to the rolls, especially if their structure has a considerable ferritic component.

What I claim is:

1. A method of obtaining a rolled alloyed steel product while preventing the adhesion of the product to the roll during rolling, which comprises immersing the semifinished and pickled product before rolling in a molten oxidising salt bath and producing thereby on the surface of the product an adherent coat mainly consisting of the trivalent iron oxide, cleaning the said coat of adhering salts, heating the coated product to hot-rolling temperature and then hot-rolling the coated product with the oxide coat acting as the parting layer between the hot roll and the product and preventing the said adhesion.

2. A method according to claim 1 in which the product is of a fer-ritic chromium steel.

3. A method of obtaining :1 rolled product of ferritic chromium steel which includes at least one of the metals selected from the group consisting of nickel and manganese while preventing the adhesion of the product to the roll during rolling, which comprises immersing the product after pickling and before rolling in a molten oxidising salt bath consisting essentially of about 10% sodium nitrate and the remainder caustic soda and producing thereby on the surface of the product an adherent coating mainly consisting of the trivalent iron oxide,

References Cited in the file of this patent UNITED STATES PATENTS 1,029,936 Maskrey June 18, 1912 1,374,711 Armor Apr. 12, 1921 1,976,299 Russell Oct. 9, 1934 2,050,298 Everett Aug. 11, 1936 2,370,959 Holden Mar. 6, 1945 2,431,986 Clingan Dec. 2, 1947 2,479,979 Spence et a1. Aug. 23, 1949 2,538,917 Sejournet et al Jan. 23, 1951 2,609,780 Whitbeck Sept. 9, 1952 2,728,696 Singer Dec. 27, 1955 2,851,420 Holden Sept. 9, 1958 FOREIGN PATENTS 863,282 Germany Aug. 2, 1956 

1. A METHOD OF OBTAINING A ROLLED ALLOYED STEEL PRODUCT WHILE PREVENTING THE ADHESION OF THE PRODUCT TO THE ROLL DURING ROLLING, WHICH COMPRISES IMMERSING THE SEMIFINISHED AND PICKLED PRODUCT BEFORE ROLLING IN A MOLTEN OXIDISING SALT BATH AND PRODUCING THEREBY ON THE SURFACE OF THE PRODUCT AN ADHERENT COAT MAINLY CONSISTING OF THE TRIVALENT IRON OXIDE, CLEANING THE SAID COAT OF ADHERING SALTS, HEATING THE COATED PRODUCT TO HOT-ROLLING TEMPERATURE AND THEN HOT-ROLLING THE COATED PRODUCT WITH THE OXIDE COAT ACTING AS THE PARTING LAYER BETWEEN THE HOT ROLL AND THE PRODUCT AND PREVENTING THE SAID ADHESION. 